Without knowning what format your tiffs are in, I can't say for sure, but they are probably just in a format not currently supported by the GDalRasterLayer. I know pallete and monochrome are not supported, but they wouldn't be hard to add.
Do you have a color image displaying as greyscale, or does it just show up as one solid color (black, gray, white)?

I'm afraid I don't know what format my TIFFs are in but they appear as a greyscale image.
If it's useful, PaintShop Pro image information shows the pixel depth/colours=8/256 and the Number of layers=1.
Actually, I quite like the option of having my Map appear without colours, especially when drawing other layers on top, but not all the time.

I am not familiar with PaintShop, but the Number of layers = 1 sounds like its just a greyscale image. A color image will probably have Number of layers = 3 (red, green, blue). You could try looking at the PaintShop info on the sample tiffs
to confirm this.

dan3258 is right, if my math holds up. Since all 8 bits per pixel are taken up by the color scale (2^8 = 256), you only have one layer. It doesn't have to be gray, if you'd prefer assigning some other color values to the scale... Not sure how you'd do this
though. Actually, Dan, does the GDALRasterLayer support mapping layer values onto a color map? I'm starting to vet the raster style stuff, and it can support this kind of styling...

Thanks for the feedback.
The image is definitely a colour one as I can see the colours correctly in both PaintShop and MapInfo.
My guess is that it uses a TIFF colour pallete which Dan says is not supported.

Works fine on Windows 2000 but gives a Gdal PInvoke error on Windows XP.
Initially I had the PInvoke error on Windows 2000 but I resolved the problem by copying all the gdal dlls from the dependencies directory to debug.
However, I've moved the software to Windows XP and it falls over on opening a TIFF with the gdal PInvoke error.
I get the same error not just with my project but with a freshly downloaded copy of DemoWinForm.
I've tested on two PCs with Windows XP and get the same error and two PCs with Windows 2000 without errors.
Can anyone suggest where to look?

I found a solution for my PINVOKE error. I downloaded the latest version of FWTools (http://home.gdal.org/fwtools/FWTools228.exe) and place the dlls that are in the /bin/ folder and the /csharp/
folder in the same folder as the /debug/ folder or equivalent folder where you have the .exe file.

this thread is quite old, the latest source in the trunk should be able to handle palette geotiffs, so my suggestion is that you get the latest source, compile sharpmap yourself and see if your issue is resolved. If that is not the case, please raise an
issue and provide the image in question.

I reverted and set userotation to true. Back to black and white. You have to set a non identity transform for the GetPreview() codepath to be followed. When i did this i didnt even get a greyscale. I got a blank screen.

The code in GetPreview looks very inefficient. The simplest and most effective way to implement paletted bitmaps is to let GDI+ handle them by converting everything to a paletted bitmap.